(a) Field of the Invention
This invention relates to a new or improved lumber sorting system and in particular to a method and an apparatus for differentiating in a reliable manner lumber pieces of separate species.
(b) Description of the Prior Art
Some lumber sorting systems based on sorting criteria other than the wood species (i.e. moisture content, density, weight) have recently been put on the market. However sorting by species rather than by the above referred to characteristics is recognized by the industry as being a more desirable practice. In particular, sorting by species is anticipated to provide more benefit in lumber drying practice than sorting by weight or moisture content, especially when spruce and fir mixed species are involved.
As has been noted by Kutscha, Lomerson and Dyer in a paper entitled "Separation of Eastern Spruce and Balsam Fir by Chemical Method" 1978, the occurrence of mixed stands of spruce and fir in some regions has led to the practice of harvesting these wood products without separation under the non-specifical label "spruce-fir". However lumber pieces of these two species have many different characteristics, particularly as concerns drying times, shrinkage potential, strength and gluing properties. The time required to dry fir lumber pieces is approximately 50% longer than that to dry spruce pieces so that if there is no separation, the kiln drying schedule must be based on the drying time for fir. This can result in over-drying of the spruce content of the kiln drying load, and because of the tendency of spruce pieces to twist, the product can suffer degradation.
While spruce and fir trees are quite easily distinguished from one another, once the trees have been reduced to lumber, spruce and fir pieces are quite similar in appearance, except for the presence of resin canals in spruce and their absence in fir. However this difference is insufficient and could not form an adequate basis for separation in any production process.
An important proportion, perhaps as much as 80% or more, of lumber currently being produced is kiln dried. One of the biggest challenges facing the industry is to minimize the energy costs of kiln drying and avoid degradation problems that can occur when drying mixed species. Many producers have adopted the use of manual sorting of wood species on a production line prior to drying, and despite the shortcomings of this method, no better technique has so far been developed.
Manual sorting suffers from several drawbacks. The first arises from the fact that the lumber pieces move along the conveyor typically at speeds up to about 175 pieces per minute, which obviously gives the sorter very little time to make any sorting decisions. Furthermore, visually distinguishing one species of wood from another is sometimes difficult. For example recent studies have indicated that the manual method provides an average success rate of only 85% in sorting spruce and fir.
U.S. Pat. No. 4,992,949 Arden describes a system for color sorting of lumber in the manufacture of products where it is important for lumber elements that are used in combination to have an homogenous appearance as regards wood color and slope of the grain. The system of Arden involves the use of a scanning camera to acquire image data and developing a histogram of frequency distribution of colour intensity for various colours. However it is not believed that this prior art system would be applicable to sorting of lumber pieces in accordance with different species of wood.
It is the aim of the present invention to provide a method and apparatus for reliably sorting lumber pieces of different species.